1. Field of the Invention
The present invention relates to a process for converting silicon metal and methanol to trimethoxysilane (TMS) and in particular to a process which is continuous and which employs multiple reactors that are serially connected.
2. Prior Art
The reaction between silicon metal and alcohol to produce alkoxysilanes and silicates is well established. As early as 1949, U.S. Pat. No. 2,473,260 described a process for the preparation of methyl silicates from methanol and silicon-copper masses. Subsequently, U.S. Pat. No. 3,072,700 taught the preparation of alkoxysilanes from silicon metal and alcohol in a fluidized bed reactor.
A patent covering the production of trialkoxysilanes is U.S. Pat. No. 3,775,457. One of the problems associated with the process of the patent is the difficulty of removing the unreacted alcohol from the desired silane because of the formation of a trimethoxysilane/methanol azeotrope and the reaction of TMS with methanol to form tetramethoxysilane.
Several patents disclose solutions to the problem of azeotrope formation. For example, Japanese Laid Open Application No. 1980-11538 describes a process to produce trimethoxysilane wherein the unreacted methanol in the product is removed by breaking the trimethoxysilane-methanol azeotrope by adding a third component, e.g., hexane, in an amount proportional to the amount of methanol present and then distilling to remove the methanol as the hexane-methanol azeotrope. U.S. Pat. No. 4,761,492, discloses a process in which methanol is separated from crude product containing trimethoxysilane using extractive distillation with tetramethoxysilane as the solvent. U.S. Pat. No. 4,999,446 discloses a process for converting silicon metal and methanol to trimethoxysilane in which the trimethoxysilane/methanol azeotrope is recycled to a reactor without employing a separate azeotropic or extractive distillation step. In these patents, the direct reaction of methanol with silicon metal takes place as "a batch process", "batchwise" or "batchwise or semicontinuously".
The direct reaction is not carried out continuously in the above-mentioned patents because fines (particles of catalyst, impurities, and/or unreacted silicon metal) build up inside the reactor. In order to remove the fines from the reactor the reaction system must be shut down. Further, in the art, silicon metal has not been fed continuously because even greater amounts of fines would form. The presence of fines in the reactor results in one or more of the following problems. Fines can cause the slurry to foam. Also, fines can act as a catalyst for the reaction of TMS and methanol, thereby producing tetramethoxysilane (TTMS), an undesirable by-product, resulting in lower yield and less pure TMS. The presence of large amounts of fines also makes the recovery of solvent by filtration difficult.
Accordingly, a need exists for a commercially attractive process for producing trimethoxysilane that allows silicon metal to be fed continuously to the reactor system and that allows for the easy removal of fines.